Device and method for electrodialysis

ABSTRACT

An improved apparatus and operating method related thereto for deionizing water to produce substantially pure water using electric field and ion exchange materials are disclosed, including embodiments incorporating one or more of the novel features of brine and electrode streams flowing in a direction counter-current to the stream being deionized, a filling of the brine stream with stratified ion exchange materials, a stream mixing feature for mixing the stream being deionized, a gas removal feature for removal of gases, a spiral-wound embodiment of an electrodialysis device according to the invention, and a method for determining the preferred operating current for electrodialysis systems according to this invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to international patent applicationPCT/US01/25226, filed Aug. 10, 2001, which in turn claims the benefit ofU.S. provisional patent application 60/224,974 filed Aug. 11, 2000 andU.S. provisional patent application 60/236,778 filed Sep. 29, 2000.

FIELD OF THE INVENTION

This invention pertains to improved electrodialysis (“ED” including“EDR”) apparatus and systems, including improved filled cellelectrodialysis apparatus and systems, and to improved processes whichuses such apparatus and systems. (Filled cell ED is also known in thisart as electrodeionization (“EDI”). Filled cell EDR is also known inthis art as reversing electodeionization (“EDIR”)).

BACKGROUND OF THE INVENTION

ED apparatus having a multiplicity of alternating anion selective andcation selective membranes was apparently first described by K. Meyerand W. Strauss in 1940 (Helv. Chim. Acta 23 (1940) 795-800). Themembranes used in this early ED apparatus were poorly ion selective. Thediscovery of ion exchange (“IX”) membranes (e.g., in U.S. Pat. No. Re.24,865) which had high ion permselectivity, low electrical resistanceand excellent stability led rapidly to the invention of ED using suchmembranes (e.g., in U.S. Pat. No. 2,636,852) and to the growth ofindustries using such apparatus, for example, for desalting of brackishwater, concentration of sea water, and deashing of cheese whey. Duringthe last 40 years approximately 5000 ED plants have been installed on aworld-wide basis.

The utility of ED continues to be limited, however, by several technicalactors, particularly relatively low limiting current densities anddeficiencies in removing poorly ionized substances. These limitationsand deficiencies of prior art ED systems are discussed further below.

A. Limiting Current Density:

Because the IX membranes used in ED are highly selective to ions of onesign or the other, a substantial fraction of the ions passing throughthe membranes must reach the membrane wails by diffusion from theambient solution through laminar flow layers which develop along theinterfaces between the membranes and the solutions being depleted ofions (the “dilute or diluting solutions or streams” as they are known inthe art). The maximum rate of diffusion of ions through the dilutingsolution occurs when the concentration of electrolyte at such membraneinterfaces is essentially zero. The current density corresponding tosuch zero concentration at a membrane interface is referred to in theart as the limiting current density. To increase the limiting currentdensity it is necessary to increase the rate of ion diffusion, forexample, by reducing the thickness of the laminar flow layers by flowingthe ambient solution rapidly by the membrane surfaces and/or by the useof turbulence promoters, and/or by increasing the temperature. Practicallimiting current densities are genera in the range of 5,000 to 10,000amperes per square meter for each kilogram-equivalent of salts per cubicmeter of solution (that is, 0.5 to 1 amperes per square centimeter foreach gram-equivalent of salts per liter). A typical brackish water has aconcentration of salts of about 0.05 kg-eq/m.³ (that is about 0.05-eq/lor about 3000 parts per million (“ppm”)), and therefore has a limitingcurrent density in the range of about 250 to 500 amperes per m.² (0.025to 0.05 amperes per cm²). In order to maximize the utilization of EDapparatus, it is desirable to operate at the highest possible currentdensities. However, as the limiting current density is approached, it isfound that water is dissociated (i.e., “split”) into hydrogen ions andhydroxide ions at the interfaces between the (conventional) anionexchange (“AX”) membranes and the diluting streams. The hydrogen ionspass into the diluting streams while the hydroxide ions pass through theAX membranes and into the adjacent solutions which are being enriched inions (the “concentrate, concentrated, concentrating or brine solutionsor streams” as they are known in the art). Because brackish water mayoften contain calcium bicarbonate, there is also a tendency for calciumcarbonate to precipitate at the surfaces of the (conventional) AXmembranes which are in contact with the concentrating streams. Thisproblem previously has been addressed by several techniques: by chemicalor IX softening of the feed waters or the concentrating streams; byadding acid to the feed waters or the concentrating streams (with orwithout decarbonation); by nanofiltration (“NF”); or, by regularlyreversing the direction of passage of the electric current therebychanging the concentrating streams to diluting streams (and the dilutingstreams to concentrating streams). See, e.g., U.S. Pat. No. 2,863,813.Of the above techniques, the most successful process has been the lastmentioned process, namely reversing the electric current, which isreferred to in the art as “electrodialysis reversal” (“EDR”).

The theory of limiting current in ED shows that in the case of sodiumchloride solution, for example, the cation exchange (“CX”) membranesshould reach their limiting current density at values which are about ⅔rds that of the AX membranes. Careful measurements have shown that suchis indeed the case. However, as the liming current density of(conventional) CX membranes is approached or exceeded, it is found thatwater is not split into hydroxide ions and hydrogen ions at theinterfaces between such CX membranes and the diluting steams. Thedifference in behavior relative to the water splitting phenomenon of(conventional) AX and CX membranes at their respective limiting currentshas been explained in recent years as catalysis of water splitting byweakly basic amines in the AX membranes. AX membranes which have onlyquaternary ammonium anion exchange groups (and no weakly basic groups)initially do not significantly split water as their limiting current isapproached. Such behavior continues for only several hours, however,after which period water splitting begins and increases with time. It isfound that the AX membranes then contain some weakly basic groups whichhave resulted from hydrolysis of quaternary ammonium groups. It isconcluded that splitting of water at conventional AX membranes at ornear their limiting current densities is an unfortunate phenomenon whichis unavoidable for practical purposes.

The existence of limiting current in ED also means that in dilutesolutions the liming current densities are relatively very low. Forexample, at a concentration of salts of about 0.005 kg-eq/m.³ (that isabout 0.005 g-eq/l or about 300 ppm, a concentration typical of drinkingwater), the limiting current density is in the range of from about 25 to50 amperes per m.² (0.0025 to 0.005 amperes per m.²), i.e., the transferof salts per unit area per unit time is very low (e.g., 50 to 100 gramsof salt per hour per square meter). This problem seems first to havebeen addressed by W. Walters et al. in 1955 (Ind. Eng. Chem. 47 (1955)61-67) by filling the diluting stream compartments in an ED stack (i.e.,a series of AX and CX membranes) with a mixture of strong base andstrong acid ion exchange (IX) granules. Since then many patents haveissued on this subject, among them U.S. Pat. Nos. 3,149,061; 3,291,713;4,632,745; 5,026,465; 5,066,375; 5,120,416; and 5,203,976, which patentsare incorporated herein by reference. Two modes of operation using suchfilled-cell ED (known as EDI) have been identified. In the first mode,the IX granules serve as extensions of the membrane surface area therebygreatly increasing the limiting current density. In the second mode, acurrent density is applied which is very much greater than the limitingcurrent density even with the presence of the IX granules. Under thesecircumstances, the rate of water splitting at membrane-diluting streaminterfaces is very high and the IX granules are predominantly in thestrong base and strong acid forms respectively. The apparatus in thismode is therefore best described as operating as continuouslyelectrolytically regenerated (mixed bed) ion exchange. An intermediatemode may also be identified in which there is some water splitting butthe IX granules are not predominantly in the strong base and strong acidforms respectively.

Most filled-cell ED (that is, EDI) systems operate in both modes, e.g.,(1) in the same ED cell, the first mode near the entrance to the celland the second mode near the et (2) in cells in flow series between asingle pair of electrodes; or, (3) in separate stacks in flow series(each stack with its own pair of electrodes). Filled-cell ED is used toreplace reverse osmosis or conventional, chemically regenerated IXsystems, e.g., a strong acid CX column followed by a weakly basic AXcolumn or, at least in part, a mixed bed IX column. In either of thelatter cases, the CX and AX granules are chemically regeneratedseparately, e.g., with aqueous acidic solutions of sulfuric acid orhydrochloric acid and aqueous basic solutions of sodium hydroxiderespectively. Precipitates of calcium carbonate, calcium sulfate andmagnesium hydroxide are thereby not obtained. The columns of finegranules are effective filters for colloid matter which is rinsed offthe granules during the chemical regeneration. In contrast, in the caseof EDI, any calcium, bicarbonate and/or sulfate removed from thediluting stream occurs in a higher concentration in the concentratingstream, particularly when it is desired to achieve high recoveries ofthe diluting stream (which is the usual case). Such higherconcentrations frequently result in precipitation in the concentratingstream. Furthermore, it is inconvenient (though technically possible) toback-wash the IX granules in a filled-cell ED apparatus thereby removingany colloidal matter which may have been filtered out.

These problems with EDI are generally solved by pretreatment processes,for example: (1) regenerable cation exchange for softening followed byregenerable anion exchange absorbents for colloid removal and/orbicarbonate removal; (2) ultrafiltration or microfiltration for colloidremoval followed by EDR for softening and partial demineralization; or,(3) ultrafiltration or microfiltration for colloid removal followed bynanofiltration for softening or reverse osmosis for softening andpartial demineralization.

As pointed out above, filled-cell ED is used to replace, at least inpart, a mixed bed IX column. The latter, however, generally produceswater having an electrical resistance of about 18 meg ohm-cm and silicaconcentrations near the present limits of detection. Such highperformance by filled-cell ED (EDI) has been difficult to achieve untilnow.

B. Removal of Poorly Ionized Substances:

ED (including EDR) is used in many plants to deash cheese whey.Generally the natural whey is first concentrated to the range of 20 to25 percent solids by weight. The current density (that is, the rate ofremoval of ash per unit area of membrane per unit time) during ED (orEDR) of such concentrated whey remains relatively high until about 50 to60 percent of the ash is removed. The remaining ash behaves as if it ispoorly ionized, perhaps associated or complexed with protein in thewhey. An important market for deashed whey requires 90 percent or higherdeashing. To deash from about a 40 percent ash level to a 10 percent ashlevel using ED (including EDR) may require much more apparatus contacttime than to deash from 100 percent to 40 percent ash. This problem maybe addressed by the more or less continuous addition of acid to the wheyduring deashing from 40 to 10 percent ash, the acid apparently freeingthe ash from the protein. However, such added acid is rapidly removed byED (including EDR), and the resulting high quantities of acid requiredto complete this process are therefore undesirable. The problem has alsobeen addressed by removing about the first 60 percent of the whey ashusing ED (including EDR) and removing most of the remaining 40 percentby ion exchange. The ion exchange apparatus for this applicationgenerally consists of a column of strong acid CX granules followed by acolumn of weak base AX granules. Considerable quantities of acid andbase are required in this process to regenerate the IX granules.

As discussed above, electrodeionization (EDI) using filled ED cells is avery useful process for removing the last traces of ionic contaminantsfrom water, but it could be significantly improved. These desiredimprovements include:

-   1) unproved product purity. The sources of impurities in EDI systems    include but are not limited to:    -   (a) Back diffusion and electromigration of ionic contaminants        through the ion exchange membranes driven by concentration        differences and electric fields;    -   (b) Back diffusion of neutral weakly ionized species through        polarized membranes; and,    -   (c) Electrodialysis of contaminant ions from membranes into the        product water in the dilute stream manifolds.-   2) Simplification of the equipment and controls needed to run    traditional EDI. Today these EDI subsystems include brine    recirculation, brine and electrolyte pH and conductivity control.    Simplification should include lowering of equipment cost, reducing    required operator expertise and shortening the time required for    monitoring and adjusting equipment.-   3) Reduction of the electric power consumption used for the EDI    stack and pumps.-   4) Improved resistance to scale formation in the concentrate    streams.-   5) Need for less pretreatment of the water before EDI.-   6) Improved product water quality without sacrificing product water    recovery.-   7) Ability to operate intermittently and to produce excellent    product immediately upon-   8) Ability to operate with elevated solution delivery pressure    thereby eliminating the need for a transfer pump.-   9) Ability to reliably operate without external leaks and without    external salt build up.

Most, if not all, of the above limitations and deficiencies ofconventional EDI systems are either overcome or at least significantlyimproved upon by the improved apparatus and methods for electrodialysisaccording to the present invention. Other objects and advantages of thepresent invention will in part be obvious and will in part appearhereinafter. The invention accordingly comprises, but is not limited to,the apparatus and related methods, involving the several steps and thevarious components, and the relation and order of one or more such stepsand components with respect to each of the others, as exemplified by thefollowing description and the accompanying drawings. Variousmodifications of and variations on the apparatus and methods as hereindescribed will be apparent to those skilled in the art, and all suchmodifications and variations are considered within the scope of theinvention.

SUMMARY OF THE INVENTION

In accordance with a first embodiment of the present invention, onemeans of achieving the desired improvements is to use the product of theEDI unit as the feed to the concentrate stream and electrode streamcompartments, and to flow the concentrate and electrode streams of theEDI unit in a flow direction substantially opposite that of the productstream in a single pass. This can be accomplished in a preferredembodiment by configuring the apparatus such that the product outletmanifold (reference numeral 3 as shown in FIG. 1A) serves as the inletmanifold for the concentrate stream (in concentrating compartments 7)and electrode stream (in electrode compartments 8). Due to the highelectrical resistance of the staring concentrate and electrode streamsthese compartments should preferably provide electrical continuity byion transport through ion exchange material. Thus, the concentrate andelectrode streams are at least park filled with an ion exchange materialor these compartments utilize ion exchange membranes with a surfacetexture that allows them to be in contact with each other at asperitieswhile allowing fluid flow around the asperities. It is preferred thatthe effluent from the concentrating compartments in the aboveembodiments of this invention not be recycled to the concentratecompartments, or, if recycled, recycled by an auxiliary entrancemanifold at an intermediate position in the concentrate flow paths.

Such internal reflux of dilute effluent to concentrate influent isadvantageously used together with periodic reversal of the direction ofelectric current through the EDI stack. If such current reversal iscontinued for a substantial period (e.g., if the reversal is essentiallysymmetric with respect to time), then it is preferred in accordance withthis invention that the direction of flow through the dilute andconcentrating compartments also be reversed, the effluent from the “new”dilute compartment then providing pure reflux to the influent to the“new” concentrating compartment.

The flow rate of dilute effluent to the concentrating compartments inthe above-described internal reflux mode is, of course, only a fractionof the effluent coming from the diluting compartments. Nevertheless, itmay be necessary and preferred to provide a back pressure for the diluteeffluent coming from the stack, such back pressure for example beingprovided by the pressure loss through a mixed bed ion exchange apparatusto which such dilute effluent is directly connected withoutrepressurization.

The concentrate effluent may, for example, be reclaimed at least in partby Reverse Osmosis, Nanofiltration, Evaporation or another stage of ED.

In order to make ultrapure water by EDI, it is necessary to remove thehighly ionized electrolytes such as NaCl, CaSO₄, etc., as well as theweakly ionized electrolytes such as CO₂, NH₃, SiO₂, and H₃BO₃. Suchweakly ionized electrolytes are only substantially removed after almostall of the strongly dissociated ions (Na+, Cl−, Ca++, SO4−, etc.) havebeen removed. Efficient removal of the weak electrolytes is obtained(generally) at very low local current efficiencies, where almost all ofthe current passing through the exchange membranes is carried by OH− andH+ respectively. If the concentrate stream is at a substantially neutralpH and not filled with an ion exchange material, then the weakly ionizedelectrolytes, such as silica, after transport through the anion exchangemembrane, become substantially non-ionized in the bulk of the brinestreams. The cation exchange membrane, although mainly preventingtransport of the negatively charged strong ions back to the dilutingstream, is unable to prevent the diffusion of low molecular weightneutral species such as CO₂, SiO₂, and H₃BO₃ back to the dilute stream.The anion exchange membrane, although preventing transport of thepositively charged strong ions back to the diluting stream, is unable toprevent the diffusion of low molecular weight neutral species such asNH₃ and amines back to the dilute stream.

The concentrate stream may have a very low conductivity, such that itrepresents more than half of the electrical resistance of the EDI stack.In such a case it may be desirable to fill the concentrate stream withan ion exchange material. The ion exchange material in the concentratestream may have cation exchange material next to the cation exchangemembrane and anion exchange material next to the anion membrane. U.S.Pat. No. 4,033,850, which is incorporated herein by reference, disclosessuch an arrangement of ion exchange material, but only for the dilutingcompartments of an electrodialysis device. The cation exchange materialjuxtaposed to the cation exchange membrane may be integral with (e.g.,texture thereon) or not integral with such membrane (e.g., as beads,rods, screen, etc.). Similarly, the anion exchange material juxtaposedto the anion exchange membrane may also be integral with such membraneor not integral with such membrane, independently of which arrangementis used with the cation exchange membrane. That is, one membrane mayhave exchange material which is integral with the membrane while theother membrane has exchange material which is not integral. Further,either or both membranes may for example be textured and in additionhave non-integral exchange material of the same charge sign juxtaposed.If both membranes are textured, the membranes may in places be in directcontact with each other with the open regions between the texturesurface projections providing fluid flow paths. In any of the aboveexamples, it will be understood that flow of solution through theconcentrating compartment must not be severely impeded.

In EDI spacers there is some maldistribution of flow through the ionexchange material due to the intersection of the sidewalls and thematerial. Because a three-dimensional ion exchange bead can notpartially penetrate the sidewalls, the packing arrangement of ionexchange material near the sidewalls is not as uniform as thearrangement in the bulk of the material away from the sidewalls, andthis steric hindrance allows more flow next to the sidewalls. Theslipstream that thus develops next to a sidewall has both a greaterdiffusion distance to, and less residence time in contact with, the ionexchange material; and, as a result, along the diluting compartmentspacer has fewer ions removed. This slipstream therefore carries agreater load of contaminants to the outlet end of the dilutingcompartment spacer where the slipstream mixes with the bulk of thespacer flow thereby decreasing the purity of the product.

In accordance with another embodiment of the present invention, however,it has been found that this effect can be minimized by adding mechanicalstatic mixers to the sidewalls of the spacers at intervals along thespacer length. These static mixers help to mix the slipstream flowingalong the sidewalls with the bulk of the flow thereby providingincreased contact time to improve the removal of the contaminants by theion exchange material. Thus, addition of the static mixers as providedherein has been found to further improve product quality. An arrangementof suitable static mixers along the spacer sidewalls of an EDI cell inaccordance with this invention is illustrated in FIG. 3.

Gas bubbles trapped in the ion exchange material or formed by outgassingof the water also may cause flow maldistribution and thereby decreaseproduct quality. Bubbles generated at the electrodes can also be trappedin the ion exchange material and grow large enough to cause poor currentdistribution in the EDI stack. All of these bubbles are difficult toremove by buoyancy or flow effects due to the small size of the ionexchange materials and the surface tension (capillary effect) attendantwrit this size range which tends to hold them in place.

In accordance with yet another embodiment of the present invention, gaspermeable (e.g., non-porous and/or hydrophobic microporous) regionsand/or elements, such as hollow fibers or other geometries, areincorporated into the EDI spacers to provide a means for the gases inthe bubbles to permeate through the regions and/or into the lumens ofthe hollow fibers and escape from the stack without loss of any liquid.An EDI cell including an arrangement of hydrophobic microporous hollowfibers in accordance with this invention is illustrated in FIG. 4. Alsoillustrated in FIG. 4 is the embodiment of providing an EDI cell withEDI spacers having hydrophobic, microporous regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic cross sectional view of a “plate and frame” typeEDI stack in accordance with this invention showing feed inlet 1, outlet4 and common manifold 3 from which the product outlet 2 emerges. Thefigure also depicts anode outlet 5, cathode outlet 6, concentratecompartments 7, electrode compartments 8, and diluting compartments 9.

FIG. 1B is a schematic top view of the EDI stack of FIG. 1A showing feedinlet 11, common manifold 13, product outlet 12, concentrate streamoutlet 14, and anode stream outlet 15. FIG. 1B also depicts theapproximate direction of the diluting stream flow 16, concentratestreams flow 17, and electrode streams flow 18. The dotted lines betweenthe various manifolds represent possible spacer shapes defining fluidflow paths between the facing walls of adjacent spacers for the threedifferent stream types, i.e., diluting, concentrating and electrode.

FIG. 2A shows a schematic cross sectional view of an alternative spiralEDI stack or module configuration in accordance with this invention withfeed inlet 21, the product outlet 22, the concentrate steam outlet 23,the diluting steams 24, the concentrate streams 25, the center electrode26 (which, in a preferred case as shown, comprises an anode), and anexterior conducting (e.g., metal) container 27 which acts as theopposite electrode.

FIG. 2B is a schematic left end view of the complete (unsectioned)spiral EDI stack of FIG. 2A showing the same entities (with the samenumbers).

FIG. 3 is a schematic plan view of a filled (EDI) spacer in accordancewith this invention showing spacer frame 31, filed in the center withion exchange material 32, and having multiple screen mixers 33 alongsidewalls 35.

FIG. 4 is a schematic plan view of a filled (EDI) spacer in accordancewith this invention showing spacer fame 41, filled in the center withion exchange material 42, and having microporous hydrophobic elements43, and/or hydrophobic microporous regions 44.

FIG. 5A is a schematic plan view of one embodiment of a textured-surfacemembrane 51 in accordance with this invention showing the raised texture52 in the center portion on one surface only. Although not shown, itwill be understood that membranes textured on both surfaces may befabricated and used in accordance with this invention.

FIG. 6 is a schematic plan view of still another type oftextured-surface membrane in accordance with this invention, thismembrane having a smaller textured pattern, such pattern havingincreased surface area and greater turbulence promotion effects.

FIG. 7 is a schematic plan view of still another type oftextured-surface membrane in accordance with this invention, thismembrane having diagonal raised stripes. It will be understood that whenthe next adjacent membrane (not shown), having die stripes in anopposite orientation, is coupled with the membrane shown in FIG. 7, theresulting EDI cell would produce a self supporting flow path withoutneed for a spacer. Such a crossed stripe pattern provides turbulencepromotion, thoroughly mixing fluid as the latter travels across alongsuch flow path.

FIG. 8A is a schematic plan view and FIG. 8B is the corresponding crosssectional view of still another membrane in accordance with thisinvention, wherein the membrane comprises 3-dimensional pleats or foldsin a flow path area. The entire body of the membrane shown in FIGS. 8Aand 8B forms a pattern.

FIG. 9A is a schematic plan view and FIG. 9B is the corresponding sideview of yet another membrane in accordance with this invention, whereinthe membrane comprises 3-dimensional waves in a flow path area.

FIG. 5B is a schematic side view of the textured-surface membrane ofFIG. 5A. FIG. 10 is a schematic cross sectional axial view of apreferred countercurrent concentrate stream spiral EDI stackconfiguration in accordance with this invention. The center “pipe” ortubular element 104 is sealed to ion exchange membranes 102 and 103, andacts also as a first electrode (anode or cathode) and is internallydivided by divider wall 107 to provide on a first side of wall 107 amanifold for feed inlet 101 and on a second side of wall 107 aconcentrate outlet 105. The product outlet 108 is not sealed to themembranes and allows product flow 109 to bypass and feed the concentratestream. Exterior shell 106 acts as the opposite electrode.

FIG. 11 is a schematic cross sectional axial view of another preferredcountercurrent concentrate stream spiral stack configuration inaccordance with this invention. The center “pipe” or tubular element 114is sealed to ion exchange membranes 112 and 113, and acts as anelectrode (anode or cathode), a product outlet manifold, and a brinefeed inlet. The concentrate outlet manifold 116 is hydraulically sealedto the edge of the oppositely charged ion exchange membrane 112 and theexterior shell 115. Feed inlet manifold 111 providing feed flow 117 issealed to the edge of the ion exchange membrane 113 and the exteriorshell 115. The exterior shell 115 also acts as the opposite electrode.

FIG. 12 is a schematic cross sectional axial view of still anotherpreferred countercurrent concentrate stream spiral stack configurationin accordance with this invention. Center “pipe” or tubular element 124acts as an electrode (anode or cathode) but is not hydraulically sealedto the ion exchange membranes, thereby allowing product flow to feed theconcentrate stream. Product outlet manifold 126 is not hydraulicallysealed to the membranes and bypass flow feeds the electrode andconcentrate streams. Concentrate outlet manifold 127 is hydraulicallysealed to the edge of ion exchange membrane 122 and to exterior shell125. Feed inlet manifold 121 providing feed flow 128 is hydraulicallysealed to the edge of the oppositely charged ion exchange membrane 123and to exterior shell 125. Exterior shell 125 also acts as the oppositeelectrode.

FIG. 13 is a schematic cross sectional view illustrating details of apreferred center electrode and manifolding device, comparable to element104 in FIG. 10, which is used to clamp and hydraulically seal themembranes 131 and screens 132 to the center electrode 133. Such deviceconsists of two arc-shaped sections of metal pipe 135 and a center wedgeentity 134. The latter compresses pipe sections 135 against each otherwith membranes or membranes and screens clamped between such sections.The center wedge entity 134 also divides the center electrode into twomanifolds and may hydraulically seal one manifold compartment from theother. Center wedge entity 134 consists of two or more pieces arrangedsuch that forcing the pieces in opposite axial directions along the axiscauses the assembly to increase in size perpendicular to such axis, thusproviding the clamping force needed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention, as shown in FIGS. 1A and 1B,uses the final product of the EDI stack to feed the concentrate streamand the electrode streams in a single pass. The preferred direction toflow the concentrate streams (in concentrating compartments 7) andelectrode streams (in electrode compartments 8) is in a directionsubstantially opposite to the direction of flow of the diluting stream(in diluting compartments 9). In this single pass embodiment, employingan initially high purity, substantially counterflow concentrate streamminimizes back diffusion and electromigration of contaminants into theproduct. The membranes located close to the product outlet are notexposed to contaminants so that these membranes contain very littlecontamination, with the beneficial result that electrodialysis and/ordiffusion of contaminants from these membranes into the product iseffectively eliminated. Due to the low conductivity of the product, theconcentrate and electrode streams are preferably provided withelectrical continuity by using ion conductive material in the respectivecompartment at least near the entrances thereto. This ion exchangematerial may be in the form of beads, granules, fibers, rods, screens,cloth, felt, fabrics, surface texturing of the membranes themselves, asherein described, etc. The ion exchange material preferably contactsboth membranes which bound the concentrate compartment and forms acontinuous contact route between these membranes at least near suchentrances. In the electrode compartment; the ion exchange materialpreferably contacts the electrode and the membrane which bound theelectrode compartment and forms a continuous contact route between them.Alternatively the electrodes may be in intimate contact with, orintegral with, the adjacent membranes. The EDI stack as shown in FIGS.1A and 1B may contain an even or odd number of cells.

The following Example illustrates the effectiveness of a single-pass EDIunit as illustrated in FIGS. 1A and 1B.

EXAMPLE

The effectiveness of an EDI apparatus using a single-pass substantiallycounter-current concentrate stream flow as shown in FIGS. 1A and 1B KnitA) was evaluated relative to a conventional EDI apparatus (Control)having substantially co-current recirculating concentrate stream flow.Both EDI stacks were assembled using the same types and numbers ofmembranes, spacers, and ion exchange resins. Table 1 below shows theremarkable improvement in performance demonstrated by the apparatus ofthe present invention (Unit A) relative to the Control unit for a brinestream in which the concentration of CO₂ is relatively high.

TABLE 1 Control Unit A Feed Conductivity (microS/cm) 20 37 Feed CO₂(ppb) 300 23,610 Brine CO₂ (ppm) 310 411 Feed Silica (ppb) 500 1,064Residence Time (sec.) 40 40 Current (Amps) 1.5 0.82 Product Resistance(Mohm-cm) 10.4 17.88

As shown in Table 1 the product quality of the single pass reverse brinestack, Unit A, is maintained even at high concentrations of carbondioxide in the brine stream. The single pass reverse brine stack is notstrongly affected by the back diffusion of carbon dioxide from the brinestream, because any back diffusion occurs at a point far upstream fromthe end of the dilute flow path, and thus can be absorbed by the ionexchange material.

The ion exchange material in the concentrate flow path may beamphoteric, cationic, anionic, a mixture of cationic and anionicmaterials, or layers or other geometric arrangements of cationic andanionic materials. In a preferred embodiment of the present invention, amaterial in the concentrate compartment consists of anion exchangematerial next to the anion exchange membrane and cation exchangematerial next to the cation exchange membrane with the two types of ionexchange materials being in contact with each other near the center ofthe compartment. The ion exchange material in the electrode stream flowpath may be amphoteric, cationic, anionic, a mixture of cationic andanionic materials, or layers or other geometric arrangements of cationicand anionic materials. In a preferred embodiment of the presentinvention, a material in the electrode stream compartment consists ofcation exchange material. Some of the cells may be bounded by ionexchange membranes of the same charge, resulting in what may be termed“neutral” cells or compartments. In one of these neutral compartments,ions pass through the compartment without changing the totalconcentration of ionized or ionizable species. The ion exchange materialin the flow path of these neutral cells may be amphoteric, cationic,anionic, a mile of cationic and anionic materials, or layers or othergeometric arrangements of cationic and anionic materials.

In order to achieve a lower electrical resistance through the ionexchange material packing in the compartments, the ion exchange materialmay be shrunk using aqueous solutions of either electrolytes, or watermiscible organics such as glycerin, propylene glycol, sugars, etc., orpartial drying before it is used to fill the EDI unit. When electrolytesare used, it may be preferable to use electrolytes (which are well knownin the art) which increase the amount of shrinkage of the ion exchangematerial. After the shrunken ion exchange material has been introducedto the EDI unit, washing it with water will expand it, resulting in agreater compressive force inside a packed compartment, and therebyresulting in essentially squashing the beads or particles of ionexchange material to obtain a greater area of contact between the ionexchange materials themselves as well as with the ion exchange membranesand the electrodes. One of the electrolytes that may be used in thisprocess step is a chloride salt. In order to avoid the generation ofchlorine in the anode compartment, and the possible oxidation of the ionexchange materials contained in said anode compartment, during theinitial startup of the stack, one preferred embodiment of this aspect ofthe present invention uses non-chloride containing salts.

Enhanced compressive force in non-polarized regions increases thecontact area between the ion exchange material and the ion exchangemembranes. This effect is beneficial because as the contact areaincreases the electrical resistance decreases. This increased contactarea thus improves the transport of ions to the ion exchange membranesand helps prevent the undesirable effect of polarization of themembranes before polarization of the ion exchange fill material. If theanion membrane polarizes before the ion exchange material polarizes,hydroxyl ions transported through the membrane cause a localized high pHin the concentrating steam. Calcium ions transported through the cationmembrane may enter this high pH area and precipitate, thus formingscale.

In a preferred embodiment of the present invention, the stack is builtwith the ion exchange materials in their fully regenerated forms. Thispreferred embodiment minimizes the diffusion of salts from the membraneareas between the solid areas of the spacer frame, which are outside ofthe active electric field, to the product manifold.

Another embodiment of the present invention uses screens or clothsconsisting of polymeric material that has been ion exchangefunctionalized at least on the surge. EDI cells filled with beads,particulate or fine fiber ion exchange packing can be used only withclean, particulate-free streams. These filled cells typically exhibithigh hydraulic resistance, trans-bed pressure gradients, and pumpingpower losses. Conventional EDIR also has the potential disadvantage of aslow approach to equilibrium after current reversal due to therelatively high ion storage capacity present in ion exchange materialsfunctionalized throughout their entire structure. Instead, byfunctionalizing only a thin layer on the outer surface of a polymericmaterial, such as polypropylene, polyethylene, etc., the approach toequilibrium after reversal is significantly faster. Using such materialin the form of a woven cloth type screen or an extruded monofilamentscreen (such as Vexar), or perforated, corrugated screen or expandedplastic screen results in a much lower hydraulic pressure drop, enablinglonger flow paths and reduced pumping power.

With all or most types of IX material there will be a discontinuity inthe density of the packing array where the array ends at a boundary,such as a spacer wall. Due to this discontinuity, a slipstream flow candevelop along such wall which may be fester than the flow through thebulk portion of the array. This slipstream flow has less residence timein the spacer and less contact with the ion exchange material. In thediluting compartment where contaminants are being removed from water,this means that the slipstream will have a higher concentration ofcontaminants, and therefore a lower electrical rice than the bulk flowthrough the bulk portion of the array. The mixing of slipstream flowwith bulk product flow at the spacer outlet will therefore increase thetotal amount of the contaminants in the total product flow and decreasethe electrical resistance of such total product flow. To ameliorate thisslipstream effect, in another embodiment of the present invention staticmixed are added along the length of the spacer wall as shown in FIG. 3.These static mixers may be placed at intervals or alternatively may fillthe entire open area of the spacer. It has been found that the effect ofsuch mixers is to mix slipstream flow with the bulk of the dilutingstream flow so that the former will have more contact time with the ionexchange material and thereby have more contaminants removed.

Such static mixers nay consist of screens, such as those made by Vexar,Inc., or woven screens, or perforated, corrugated or expanded screens.The static mixers may be made of polyethylene, polypropylene, or anyinsoluble material, and are preferably attached to the spacer wall. Inanother variation, such screens may also be made at least in part of ionexchange materials. The screens may protrude at intervals into the bulkion exchange material or may extend across the entire spacer area toprovide mixing of the slipstream with the bulk flow. In a preferredembodiment of this aspect of the present invention, the screens arerecessed into the spacer wall and divert the flow to be mixed into thescreen space in the wall.

The spacing of the screen strands may be between 0.01 inches (0.254 mm)and 1.0 inches (25.4 mm), preferably between 0.0625 inches (1.59 mm) and0.5 inches (12.7 mm), and most preferably between 0.1 inches (2.54 mm)and 0.25 inches (6.35 mm). The optimum thickness of the screens and thespacing of the static mixers along the spacer walls are dependent inpart on the dimensions and geometry of the ion exchange material. Forion exchange beads or granules with a mean diameter of about 0.5 mm, thescreen thickness might advantageously be between 0.001 mm and 10 mm. Ifthe screen extends throughout the entire spacer, its strands should bethin enough and spaced far enough apart as to not significantlyinterfere with the contacts between the ion exchange materials unless atleast the surfaces of the screens are made of an ion exchange material.

Gas bubbles can be trapped in the ion exchange materials, be generatedat the electrodes, or be formed by outgassing of the aqueous stream.Small gas bubbles trapped between beads of ion exchange material cancause significant variation in hydraulic permeability and flow in theEDI spacers. These small bubbles may grow to a size at which they canresult in poor current distribution and, in the extreme case, causemembrane burning or electrode failure. U.S. Pat. No. 5,558,753, which isincorporated herein by reference, discloses means for gas removal in theconcentrating steam recycle loop.

Gas bubbles may also be trapped in diluting stream, concentrating streamor electrode stream compartments of an EDI stack. In accordance withstill another aspect of the present invention, it has been found thatgas-permeable materials can be used to allow trapped gases to permeateout of such EDI compartments. Such gas-permeable materials may behydrophobic and microporous so that the gases permeate through the poresof the material, or they may be nonporous (e.g., silicone rubber) sothat the gases diffuse through the body of the material. If the materialis hydrophobic and microporous, the pores must be of small enougheffective diameter so that surface tension prevents penetration ofliquid into the pores at the operating pressures of the EDI apparatusresulting in the loss of liquid. In one embodiment of this aspect of thepresent invention, these materials may be incorporated into the EDIspacers in the form of gas-permeable regions, hollow fibers and/or othergeometries as shown in FIG. 4. These gas-permeable materials may besupported by other materials on the side of the gas-permeable materialopposite the side facing the aqueous stream.

Another preferred embodiment of the present invention provides the EDIunit in the form of a spiral-wound element, with the product outletfeeding the inlet of a single-pass concentrate stream as genera shown inFIGS. 2, 10, 11, 12, and 13. In contrast to U.S. Pat. No. 5,376,253,which discloses a configuration wherein there is a tight seal separatingthe dilute and concentrate streams, in this embodiment of the presentinvention, the dilute effluent and concentrate influent streams share acommon manifold. Thus, in the spiral-wound configuration of thisinvention, the diluting stream may optionally be fed to either the anodecompartment or the cathode compartment before passing through thediluting stream compartment. Furthermore, a portion optionally may exitas product just prior to the electrode compartment or after passingthrough the opposite electrode compartment. The spiral-wound element ofthe present invention may consist of a single pair of membranes (cationexchange and anion exchange) wound together (as illustrated in theFigures), or it may consist of two pairs or multiple pairs (aconfiguration which is readily understood but not shown in the Figures).In either case, the spiral winding defines a (virtual) central axisabout which the membranes are wound to create the appearance of a spiral(when viewed endwise). The flow of liquid through the concentratingcompartment(s) may be inward toward the virtual axis or alternativelyflow may be outward away from such axis. In accordance with thisembodiment of the present invention, the flow of liquid through thediluting compartment(s) is then respectively outward or, alternatively,inward. In still another variation using the spiral configuration, theflow of liquid through the concentrating compartment(s) may instead begenerally “parallel” to the virtual axis (e.g., in a direction from leftto right) and, alternatively “anti-parallel” (which is defied herein asbeing the opposite of the “parallel” direction, in this example fromright to left) with the corresponding flow through the dilutingcompartment(s) being countercurrent, i.e., respectively “anti-parallel”and, alternatively, “parallel.” In other words, in this embodiment flowmay be generally parallel to the virtual axis in a first direction and,alternatively, generally parallel to the virtual axis in a second,opposite direction.

In any of the above cases, the electric current through the spiral maybe reversed thereby resulting in the “old” concentrating compartmentsbecoming “new” diluting compartments, and, respectively, the “old”diluting compartments becoming “new” concentrating compartments. Aftersuch current reversal, the direction of flow through the compartments isreversed in accordance with this embodiment of the invention, with the“new” diluting compartments providing counter-flow of “pure” effluent tothe “new” concentrating compartments.

The preferred current to apply to the EDI unit configured and operatedin accordance with this invention may be determined by plotting thefollowing equation:R_(prod)vsI/q(C_(in)−C_(out))where

-   -   R_(prod) is the electrical resistance of the product of the        diluting cells,    -   I is the current applied to the EDI unit,    -   q is the volumetric flow rate to the diluting cells,    -   C_(in) is the concentration of ionized or ionizable species in        the solution fed per diluting    -   cell in equivalents per unit volume, and    -   C_(out) is the concentration of ionized or ionizable species in        the solution exiting per diluting cell in equivalents per unit        volume.

The plot of this data will have an inflection point. The preferredcurrent to apply to the EDI unit should be a current above thisinflection point R_(prod), I, q, C_(in), and C_(out) may be expressed inany compatible units, as the object of interest is the inflection point.Furthermore, it should be noted that a line fitted to data points belowthe inflection point has one slope, whereas a line fitted to data pointsabove the inflection point has a second, different slope. The inflectionpoint corresponds to the point on the data plot at which the absolutevalue of the second derivative (d² R_(prod)/d (I/q(C_(in)−C_(out)))²) isa maximum. This means that if a regression formula is used to fit thedata to the plot, it must be such that the second derivative is still avariable of I/q(C_(in)−C_(out)). Alternatively, instead of plottingR_(prod) vs I/q(C_(in)−C_(out)), one can plot any of the following datacorrelations to determine the inflection point:R_(prod)vsi/q(C_(in)−C_(out))R_(prod)vsi/q(c_(in)−c_(out))R_(prod)vsi(R_(in)R_(prod))/q(R_(prod)−R_(in))R_(prod)vs(R_(in)R_(prod))/qv_(p)(R_(prod)−R_(in))where R_(in) is the electrical resistance of the feed to the dilutingcompartments, i is the average current density, c_(in) is theconductivity of the feed to the diluting compartments, and C_(out) isthe conductivity of the effluent from the diluting compartments. It willbe clear to one skilled in the art that it is preferable to select fromthe above alternative data correlations one which produces a sharpchange in slope near the inflection point. Further it will be understoodthat either side of any of the above correlations can be multiplied ordivided by a constant without altering the inflection point value. Themultiplication or division constant need not be the same for each sideof a correlation as the impact will be understood to expand or shrinkone, or the other, or both axes. In addition, a constant may be added orsubbed from either side, which will shift the graphical location of theinflection point with altering its value. For example, this means thatin an actual plot of the data, the axes may be individually expanded orcontracted or offset if that makes it easier or more convenient fordetermining the inflection point. The correlation data can bemanipulated in other ways well known in the art if desired.

Another aspect of the present invention is a method of automaticallycontrolling the current sent through the EDI unit according to the ionicload being fed to the unit. It is desirable from an efficiencystandpoint to send more current through the unit at higher ionic loads,and less current as the ionic load decreases. The automatic control isachieved, according to this aspect of the present invention bymonitoring the EDI feed stream by means of an associated conductivitycell/meter and using the (preferably temperature-corrected) output ofthe conductivity meter to automatically adjust the amount of currentsent to the EDI unit. This embodiment therefore minimizes the averagepower consumption and improves the overall unit performance.

Efficient capture of silica and boric acid from aqueous solutions usingelectrodialysis typically requires large amounts of anion resin in theregenerated form. It has been found that the presence of carbonate in adiluting stream significantly reduces the amount of such anion resinavailable, and therefore the efficiency of silica and boric acid captureis greatly reduced by the presence of carbonate. The current efficiencyof removing all but the first portion of carbonate is very poor becausethe bulk of the resin is then in the OH⁻¹ form, and the mobility of OH⁻¹is about 3 times higher than carbonate for a specific resin. When anionresin is in the 50% carbonate and 50% hydroxide form, it takes 8electrodes to remove one CO₂ molecule, that is 6 to move OH⁻¹, and 2 tomove the CO₃ ⁻². When anion resin is in the 20% carbonate and 80%hydroxide form, it takes 26 electrons to remove one carbonate with 24 ofthese “wasted” on moving OH⁻¹. CO₂ however can be very efficientlyremoved as bicarbonate using only one electron per CO₂ if the resin isnot highly polarized. (Despite the HCO₃ ⁻¹ having only about 20 percentof the mobility of OH⁻¹, there are virtually no OH⁻¹ ions present ifHCO₃ ⁻¹ is present in significant concentrations.)

Operating the stack with the current automatically controlled on a realtime basis relative to the ionic load, according to this aspect of thepresent invention will result in the CO₂ removal occurring primarily asbicarbonate with correspondingly greater efficiency, in essentially thesame location in the stack. While miming the average power consumption,this mode of operation leaves a maximum amount of the stack's anionresin in the desirable OH⁻¹ form.

This phenomenon can be best understood by an example: If the current isconstant, and the anion load is doubled for a brief time, CO₂ would becaptured and removed from the resin much closer to the stack outlet end.At the high anion load, the CO₂ will be efficiently removed asbicarbonate, but the silica removal capability of the stack will beseverely compromised by the reduction in the hydroxide form of the resinavailable to capture silica. Moreover, this problem can be exacerbatedby the fact that there can be a very significant transient release ofsilica into the product water because the carbonate, and thenbicarbonate, both displace the silicate that was present in the anionresin. Even after the high anion load transient is completed, thesituation does not improve. The HCO₃ ⁻¹ saturated resin quicklypolarizes, and the efficiency of electrical removal of the resultingcarbonate plunges rapidly. Thus, it takes a very long time with lots ofcurrent before the efficient capture of silica can be reestablished inthis system. As can be seen by extrapolating this example, continuousfluctuation in the current efficiency of operation of the stack willresult in poor silica removal that is nearly as bad as if it wereoperated continuously at the lowest instantaneous current efficiency.

It has now been found in accordance with another embodiment of thepresent invention that a feedback loop from the feed conductivity (orconductivity, flow and alkalinity) and/or stack electrical impedance tocontrol current, can greatly improve system performance. Such electriccurrent control may be advantageously effected by incorporatingsegmented electrodes in the electrodialysis stack. Either or bothelectrodes may be segmented permitting the current (density) at variousregions in the flow paths of the dilute compartments to be fine tuned inaccordance with the ionic load to such compartments. These segmentedelectrodes can also be used to determine the impedance at variousregions along the length of the flow paths. This impedance informationcan be used to determine the relative state of the ion exchangematerials (i.e., fully regenerated ion exchange materials have a lowerresistance than those in a salt form) and to automatically adjust thecurrent through specific segments to fine time the current (density) toindividual segments in accordance with the ionic load.

Furthermore, the nature (type) of the anion exchange resin, whether asmaterial non-integral with the anion exchange membranes or, at least inpart integral with such membranes, may be varied along the flow path. Byway of illustration, in order of decreasing basicity, the common anionexchange moieties may be aged approximately as follows:

“Type I”, poly(N-vinyl benzyl-N,N,N,-trimethyl ammonium)

“Type III”, poly(N-vinyl benzyl-N-(3-hydroxy propyl)-N,N-dimethylammonium)

“Acrylic quaternary”, poly(N-acrylamidopropyl)-N,N,N-trimethyl ammonium)

“Type II”, poly(N-vinyl benzyl-N-(2-hydroxy ethyl)-N,N-dimethylammonium)

“Weak base”, poly (N-acrylamido propyl)-N,N-diethyl amine.

The latter anion exchange resin (formula 5 above) appears to be the mostbasic of the so-called weak base anion exchange resins. In its free basefor, it is sufficiently basic to form a salt with CO₂ (apparentlyabsorbed primarily as HCO₃ ⁻¹ and not CO₃ ⁻²). It is not, however,sufficiently basic to form a salt with silica.

At the upper end of the above scale, Type I anion exchange (“AX”) resins(formula 1 above) and their equivalents in the hydroxide form are verystrong bases, very able to absorb silica and boric acid and, as notedabove, absorbing CO₂ primarily as CO₃ ⁻² and not HCO₃ ⁻¹).

The internal pK_(b) (the negative logarithm of the base dissociationconstant) is reported in the literature for the above resins. It hasbeen found that the ratio of the fraction of electrical current carriedby absorbed free and combined carbon dioxide to the fraction of currentcarried by hydroxide ions is greatest for the weak base resin at thebottom of the above list (formula 5) and smallest for the very strongbase resin at the top of the list (formula 1). It is advantageous,therefore, according to this embodiment of this invention to use resinsfrom the lower part of the above list (e.g., formulas 4 and 5 or theirsubstantial equivalents) to localize and control removal of free andcombined carbon dioxide in a region of the flow paths of the dilutecompartments in the vicinity of the flow entrances thereto, and to useresins from the upper part of the above list (e.g., formulas 1, 2 and 3or their substantial equivalents) to localize and control removal ofsilica and/or boric acid in a region of the flow paths of the dilutecompartments in the vicinity of the flow exits therefrom.

Although the above listed Type II resin and weak base resin arefrequently regarded as “intermediate base” resins, such term is usuallyreserved for anion exchange resins deliberately containing quaternaryammonium groups and non-quaternary amine groups. Such resins may bemanufactured as such or prepared by controlled degradation ofappropriate quaternary ammonium resins. The wide variety of such resinsmakes it difficult to include them in the above list relative to thelisted resins, but one skilled in the art can easily determine theuseless of any specific intermediate base or other resin for the presentembodiment of this invention.

In particular, ED or EDR benefits from an uneven texture or a raisedresin on the surface of ion exchange membranes because such a surface ofproper geometry creates a more turbulent flow across the membranesurface and thus reduces the formation of stagnant ion-depleted regionsnear the membrane surface, in addition to creating more surface area, ascan clearly be seen in FIGS. 5, 6, 7, 8 and 9. The formation ofstagnant, ion-depleted regions near the membrane surface leads to aphenomenon termed “concentration polarization,” which adversely affectsthe performance of ED and EDR systems. The textured or raised surfacemembranes of this embodiment of this invention may be used in anyelectrodialysis compartment where the process benefits from an increasein turbulent flow across the membrane and/or increased membrane surfacearea, which surface area includes channels between the membranes orbetween a membrane and an electrode.

The ion exchange membranes according to this embodiment of the presentinvention are advantageously patterned so that several layers areproduced. For example, in one premed form a flow path is produced at onedepth and a textured surface is produced between the flow paths. On theperimeter of the membranes, the surfaces are advantageously lower toallow for a nonconductive or conductive frame between the edge of themembrane and the next adjacent membrane. In this way an ED, EDR, EDIR orEDI stack can be constructed without using separate “spacers” to producethe flow path between membranes for the liquid to be treated.

The novel membranes of this aspect of the invention may also be used ina non-electrical system applications where ion exchange, ion capture, orneutralization takes place between ions in fluid and solid ion exchangemedia having a high surface area. Such membrane applications include butare not limited to use as a demineralizer, a water softening media, pHadjusting media, and metal selective or ion selective media (nitrateselective or monovalent ion selective).

In accordance with this aspect of the present invention, an uneventexture or a raised resin is imparted to the surface of ion exchangemembranes, thereby creating a significantly greater surface area for usein EDI, EDIR, ED, EDR or other electrically driven processes that useion exchange membranes and would benefit from having a non-flat surfaceas illustrated in FIGS. 5, 6, 7, 8 and 9. These textured or raisedsurface membranes can be fabricated using standard membrane formulationsfor ion exchange membranes. These membranes can also be fabricated ascharge-selective (monovalent ion selective) membranes orspecies-selective membranes, such as heavy metal-selective,nitrate-selective or sodium-selective membranes.

A particularly advantageous use for the raised surface membranesdescribed herein is in EDI applications in place of ion exchange resinsthat are usually placed between two membranes or between membranes andelectrodes. Any section of an EDI stack where an ion exchange resinpositioned next to the same charge ion exchange membrane is presentlyused may advantageously be replaced with a raised surface membraneaccording to this invention. Also, such membranes may be used in anyelectrodialysis compartment that benefits from an increase in turbulentflow across the membrane and/or from increased membrane surface area,i.e., through channels between membranes or between membranes andelectrodes.

Such surface textured membranes may also advantageously be used in anEDI stack to improve the contact area between ion exchange materials infled cells and the surface of the membrane. As previously discussed,such increased contact area decreases the cell electrical resistance andimproves the transport of ions from the ion exchange filler material tothe ion exchange membranes.

One textured surface membrane embodiment of the present invention isillustrated in FIGS. 5A and 5B, where a membrane is shown with atextured surface on only one side of the membrane sheet so that it canbe placed against the smooth surface of the next membrane, or against anion exchange material used to produce a filled cell. In a first case,the textured surface of the membrane provides a flow path means forliquid to flow between the membranes without the need for a spacer orscreen, which would normally be used to provide a hydraulic flow path.In a second case, the textured surface of the membrane will also providea greater contact area between the ion exchange filler material and themembrane.

In an alternative embodiment, a membrane is provided with a texturedspice on both sides. In one case, in conjunction with a textured surfaceof an adjacent membrane, a flow path is provided for liquid passagebetween the membranes without the need for a spacer or screen, whichwould normally be used to provide the hydraulic flow path. In anothercase, the textured surfaces of two membranes bound a filled cellcontaining an ion exchange material and provide a greater contact areabetween the ion exchange material and the membranes. In said anotheralternative embodiment; every other membrane in a stack may be texturedon both sides, and the intervening membranes are not textured on eitherside.

In other preferred embodiments of this aspect of the invention, forexample as shown in FIGS. 6 and 7, raised surface membranes may befabricated so as to provide a defined flow path in the membrane surfaceon one or both sides of the membrane. This flow path may be of anydesired shape, and may have smooth walls and bottom, or shapes may bemade in the flow path to promote turbulence, or a greater contact withan ion exchange material that may be used to fill the flow path.

Textured or raised surface membranes in accordance with this aspect ofthe invention may advantageously be fabricated in various ways. Onemethod involves the use of a patterned surface release layer to impartthe desired texture to the membrane surface. The pattern on the releaselayer can be produced in many ways, such as molding, embossing, vacuumforming, etc. The release layer may be reusable or disposable dependingon the cost and durability of the release layer material.

Another such method uses a patterned screen or other patterning layeroutside of the release layer to form the textured surface on themembrane. The release layer in this case must be pliable enough to allowthe membrane monomer to comply with the pattern under sufficientpressure to form the textured surface on the membrane.

The above-described methods may be used to fabricate membranes that aresubstantially flat but have a textured surface. In other embodiments ofthis aspect of the invention, the entire membrane may be molded orotherwise formed as a three dimensional shape. The edges of the membranemay be made flat for sealing purposes while the flow path areas of themembrane may be molded as a convoluted eggcrate, pleats, waves, bumpsand valleys, etc. FIGS. 8 and 9 show some possible forms that thesetypes of membranes may take. FIG. 8 depicts an accordion-pleatedmembrane, with grey exaggerated decisions to the pleats for illustrationpurposes. The pleats should range from about 0.010 inches to about 0.5inches. Similarly FIG. 9 depicts a wave-pattern membrane. Both pleatedand wave-type patterns may be placed at angles ranging from about 10degrees to about 80 degrees, preferably about 45 degrees, relative tothe direction of the fluid flow. By placing the next membrane adjacentwith the pleats or waves running in the opposite direction in contactwith the pleats or waves of the first membrane, a flow path is therebyformed for the fluid between the membranes that provides significantturbulence promotion.

The membranes used in accordance with this aspect of the presentinvention may be reacted from the monomeric species to the substantiallyfully polymerized form, with the pattern-creating method in place duringsubstantially the entire time. Alternatively, the membrane may bepartially polymerized to produce what is commonly known in the art as a“prepreg” material, which can then be imprinted with a preferred patternand thereafter substantially fully polymerized. A membrane having suchaforesaid shapes or textures may be molded or formed using vacuum orpressure combined with heat from ion exchange resin mixed with apolymeric binder. This type of membrane is known in the art as a“heterogeneous membrane”.

It is well known in the ED art that polarity reversal allows removal ofmaterials that are difficult to transport through a membrane, includinglarge ions and organic material, from the dilute stream with thensubsequent release into the brine stream on the reverse cycle. Thiscleaning action greatly enhances the system operating time betweencleanings. ED/EDI stacks that utilize the “reverse brine” concept inaccordance with this invention are inherently better suited for polarityand flow reversal than conventional ED/EDI stacks that use arecirculating brine stream. In the standard ED/EDI stacks utilizing arecirculating brine stream, the ion exchange materials in theconcentrating compartments are substantial in the salt form. When the DCelectrical polarity is reversed, causing the concentrating compartmentsto become the diluting compartments, some of the salt is lost into theproduct stream until the downstream portion of the ion exchangematerials becomes substantially regenerated by the hydrogen and hydroxylions generated by water splitting. During the time required for thisregeneration to occur, the resistivity of the product is substantiallower (because of increased ion content) than during normal operation,so that this portion of product water must be discarded or recycled. Ifthis water portion were blended into the normal product, it wouldproduce an overall lower resistivity and lower quality product.

By contrast, in the reverse brine stacks according to the presentinvention, the part of the ion exchange materials near the feed inletsto the diluting compartments, and the part near the outlets of theconcentrating compartments, are mostly in the salt form; and, the partof the ion exchange materials nearer to the outlet of the dilutingcompartment, and that nearer to the inlet of the concentratingcompartment, are mostly in the regenerated form. Thus, when the DCelectrical polarity is reversed (and the concentrating compartmentsbecome the diluting compartments, and the flows are reversed indirection in both types of compare), the resistivity of the productwater is maintained at substantially the same level as during normaloperation. This represents a substantial improvement over conventionalED/EDI systems. This polarity reversal can take place frequently, suchas several times per hour or less frequency such as daily or even everyfew months. This reversal can be accomplished automatically or manuallyby means of appropriate valves, or can be accomplished by manual“replumbing” of the systems. Chemical release agents including salts,acids, bases, and/or nonionic detergents may be added to the brinestream. In a preferred embodiment an additional inlet means may beprovided at a point in the brine stream downstream of the brine inletmeans for the introduction of aforesaid chemical release agents.

In accordance with another aspect of this invention described herein, anuneven membrane texture or raised resin shapes are imparted to one orboth surfaces of ion exchange membranes, creating a significantlygreater surface area for use in EDI, ED, EDR or other electricallydriven processes, which use ion exchange membranes and benefit fromhaving a non-flat surface, as well as for use in standard ion exchangeprocesses. The raised surface membranes are used in EDI together with,or in place of, ion exchange resins that are usually placed between twomembranes or between membranes and electrodes. Any section of an EDIstack where an ion exchange resin next to the same-charge ion exchangemembrane is presently used can be advantageously replaced with atextured or raised surface membrane. Also, these types of membranes maybe advantageously used in any electrodialysis compartment that benefitsfrom an increase in turbulent flow across the membrane and/or increasedmembrane surface area or through the channels between membranes andelectrodes. These surface textured membranes are also advantageouslyused in an EDI stack to improve the contact area between ion exchangematerials in the filled cells and the membrane. Increased contact areadecreases cell electrical resistance and improves the transport of ionsfrom the ion exchange filler material to the ion exchange membranes.

It will be apparent to those skilled in the art that other changes andmodifications may be made in the above-described apparatus, processesand methods without departing from the scope of the invention here, andit is intended that all matter contained in the above description shadbe interpreted in an illustrative and not a limiting sense.

1. An electrodialysis apparatus comprising a membrane stack defining aplurality of diluting compartments, alternating with a plurality ofconcentrating compartments, whereby each diluting compartment isadjacent to a concentrating compartment along at least one dilutingcompartment side, and each concentrating compartment is adjacent to adiluting compartment along at least one concentrating compartment side,diluting compartment and concentrating compartment spacers, at least anelectrode compartment, at least an electrode pair for establishing anelectric current across said stack, and inlets and outlets for flowingliquids to or from the stack, said stack further comprising at least onesubsystem selected from the group consisting of: a) a manifold systemcomprising at least a product outlet manifold which is internal to saidstack and intersects a plurality of the compartments, said productoutlet manifold being configured to receive product streams from aplurality of the diluting compartments in said stack and also to serveas an internal inlet manifold for feeding effluent portions of saidproduct streams to a plurality of the concentrating compartments in saidstack where the manifold intersects such concentrating compartmentswithout said effluent portions leaving the stack whereby portions ofproduct streams from a plurality of diluting compartments in said stackflow into and through a plurality of concentrating compartments in saidstack in a flow direction that is substantially opposite to a directionof fluid flow in said diluting compartments; and, b) a manifold systemcomprising at least a product outlet manifold which is internal to saidstack and intersects a plurality of the compartments, said productoutlet manifold being configured to receive product streams from aplurality of diluting compartments in said stack and also to serve as aninternal inlet manifold for feeding effluent portions of said productstreams to one or more electrode compartments in said stack where themanifold intersects such electrode compartments without said effluentportions leaving the stack whereby portions of product streams from aplurality of diluting compartments in said stack flow into and throughone or more electrode compartments in said stack.
 2. An electrodialysisapparatus according to claim 1(a) further wherein said product outletmanifold is further configured to serve as an internal inlet manifoldfor also feeding a portion of said product streams to at least oneelectrode compartment without said portion leaving the stack.
 3. Anelectrodialysis apparatus according to claim 1(a), wherein saidapparatus comprises two electrode compartments, and further wherein saidproduct outlet manifold is configured to serve as an internal inletmanifold for also feeding a portion of said product streams to each ofsaid electrode compartments without said portion leaving the stack. 4.An electrodialysis apparatus according to claim 1 wherein, in one ormore compartments, cation exchange material is juxtaposed to cationexchange membrane and anion exchange material is juxtaposed to anionexchange membrane.
 5. An electrodialysis apparatus according to claim 4wherein the cation exchange material is not integral with the cationexchange membrane.
 6. An electrodialysis apparatus according to claim 4wherein the anion exchange material is not integral with the anionexchange membrane.
 7. An electrodialysis apparatus according to claim 4wherein the cation exchange material is not integral with the cationexchange membrane and the anion exchange material is not integral withthe anion exchange membrane.
 8. A process for removing ionized and/orionizable substances from a liquid containing such ionized and/orionizable substances, the process comprising the steps of: (A) providingan electrodialysis membrane stack defining a plurality of dilutingcompartments, alternating with a plurality of concentratingcompartments, whereby each diluting compartment is adjacent to aconcentrating compartment along at least one diluting compartment side,and each concentrating compartment is adjacent to a diluting compartmentalong at least one concentrating compartment side, at least an electrodecompartment, at least an electrode pair for establishing an electriccurrent across said stack, and inlets and outlets for flowing liquids toor from the stack, said stack further comprising at least one subsystemselected from the group consisting of: a) a manifold system comprisingat least a product outlet manifold which is internal to said stack andintersects a plurality of the compartments, said product outlet manifoldbeing configured to receive product streams from a plurality of dilutingcompartments in said stack and also to serve as an internal inletmanifold for feeding effluent portions of said product streams to aplurality of concentrating compartments in said stack where the manifoldintersects such concentrating compartments without said effluentportions leaving the stack whereby portions of product streams from aplurality of diluting compartments in said stack flow into and through aplurality of concentrating compartments in said stack in a flowdirection that is substantially opposite to a direction of fluid flow insaid diluting compartments; and, b) a manifold system comprising atleast a product outlet manifold which is internal to said stack andintersects a plurality of the compartments, said product outlet manifoldbeing configured to receive product streams from a plurality of dilutingcompartments in said stack and also to serve as an internal inletmanifold for feeding effluent portions of said product streams to one ormore electrode compartments in said stack where the manifold intersectssuch electrode compartments without said effluent portions leaving thestack whereby portions of product streams from a plurality of dilutingcompartments of said stack flow into and through at least one electrodecompartment of said stack; and, (B) flowing said liquid into said stackand applying electrical current to said stack.
 9. A process according toclaim 8 wherein, in one or more compartments, cation exchange materialis juxtaposed to cation exchange membrane and anion exchange material isjuxtaposed to anion exchange membrane.
 10. A process according to claim9 wherein the cation exchange material is not integral with the cationexchange membrane.
 11. A process according to claim 9 wherein the anionexchange material is not integral with the anion exchange membrane. 12.A process according to claim 9 wherein the cation exchange material isnot integral with the cation exchange membrane and the anion exchangematerial is not integral with the anion exchange membrane.
 13. Anelectrodialysis apparatus comprising a membrane stack defining aplurality of diluting compartments, alternating with a plurality ofconcentrating compartments, whereby each diluting compartment isadjacent to a concentrating compartment along at least one dilutingcompartment side, and each concentrating compartment is adjacent to adiluting compartment along at least one concentrating compartment side,diluting compartment and concentrating compartment spacers, at least anelectrode compartment, at least an electrode pair for establishing anelectric current across said stack, and inlets and outlets for flowingliquids to or from the stack, said stack further comprising a manifoldsystem comprising at least a product outlet manifold which is internalto said stack and intersects a plurality of the compartments, saidproduct outlet manifold being configured to receive product streams fromat least two diluting compartments in said stack and also to serve as aninternal manifold for feeding effluent portions of said product streamsto at least two concentrating compartments in said stack where themanifold intersects such concentrating compartments without saideffluent portions leaving the stack whereby portions of product streamsfrom at least two diluting compartments in said stack flow into andthrough at least two concentrating compartments in said stack in a flowdirection that is substantially opposite to a direction of fluid flow insaid diluting compartments.
 14. An electrodialysis apparatus accordingto claim 13 further comprising cation exchange membranes and anionexchange membranes wound together in spiral form, together formingspiral diluting compartments and spiral concentrating compartments. 15.An electrodialysis apparatus according to claim 14 further comprisingliquid entrances and liquid exits arranged to effect flow of liquid insaid one or more spiral diluting compartments inwardly in a spiral oralternatively outwardly in a spiral.
 16. An electrodialysis apparatusaccording to claim 14, wherein said one or more spiral dilutingcompartments define a central axis, said stack also having one or moreliquid entrances and exits arranged to effect flow of liquid in said oneor more spiral diluting compartments in a direction substantiallyparallel to said central axis.
 17. An electrodialysis apparatusaccording to claim 14, wherein said one or more spiral concentratingcompartments define a central axis, said stack also having one or moreliquid entrances and exits arranged to effect flow of liquid in said oneor more spiral concentrating compartments inwardly in a spiral oralternatively outwardly in a spiral.
 18. An electrodialysis apparatusaccording to claim 14, wherein said one or more spiral concentratingcompartments define a central axis, said stack also having one or moreliquid entrances and exits arranged to effect flow of liquid in said oneor more spiral concentrating compartments in a direction substantiallyparallel to said central axis.
 19. An electrodialysis apparatusaccording to claim 13 further comprising an electric current switchmechanism for reversing a direction of electric current being directedacross said stack.
 20. An electrodialysis apparatus according to claim13 further comprising a fluid flow control mechanism for periodicallyreversing fluid flow directions in said diluting compartments and saidconcentrating compartments of the stack.
 21. An electrodialysisapparatus according to claim 13 further wherein a diluting compartmentcomprises a cation exchange membrane and an anion exchange membranehaving ion exchange material juxtaposed to said cation exchange membraneand to said anion exchange membrane, at least said ion exchange materialjuxtaposed to said anion exchange membrane comprising anion exchangematerial, said anion exchange material located in regions adjacent todiluting compartment entrances being effective to remove free andcombined (available) carbon dioxide from liquid entering said dilutingcompartments when such combined carbon dioxide is substantially onlybicarbonate.
 22. An electrodialysis apparatus according to claim 21wherein said anion exchange material in regions adjacent to saidcompartment entrances is not integral with said anion exchange membrane.23. An electrodialysis apparatus according to claim 21 wherein saidanion exchange material in regions adjacent to said compartmententrances is at least in part integral with said anion exchangemembrane.
 24. An electrodialysis apparatus according to claim 13including an electric current control system for maintainingpredetermined removal of silica and/or boric acid in said dilutingcompartments which comprises at least one segmented electrode and acontrol mechanism responsive to the ionic load for controllingelectrical currents to said at least one segmented electrode.
 25. Anelectrodialysis apparatus according to claim 13 including an electriccurrent control system for maintaining removal of free and combinedcarbon dioxide in the predetermined regions which comprises at least onesegmented electrode and a control mechanism responsive to the ionic loadfor controlling electrical currents to said at least one segmentedelectrode.
 26. An electrodialysis apparatus according to claim 13comprising a membrane stack defining a plurality of dilutingcompartments, alternating with a plurality of concentratingcompartments, diluting compartment and concentrating compartmentspacers, at least an electrode compartment, at least an electrode pairfor establishing an electric current across said stack, and inlets andoutlets for flowing liquids to or from the stack, said stack furthercomprising diluting compartments and concentrating compartmentscomprising at least one membrane having a surface texture facing atleast part of at least one flow path in at least some of said dilutingand concentrating compartments, said surface texture being effective toestablish substantial contact with an adjacent membrane, said adjacentmembrane having or not having such a surface texture.
 27. Anelectrodialysis apparatus according to claim 13 wherein said productoutlet manifold which is internal to said stack comprises a set ofaligned apertures in the membranes comprising said membrane stack. 28.An electrodialysis apparatus according to claim 13 further wherein saidproduct outlet manifold is further configured to serve as an internalinlet manifold for also feeding a portion of said product streams to atleast one electrode compartment without said portion leaving the stack.29. An electrodialysis apparatus according to claim 13, wherein saidapparatus comprises two electrode compartments, and further wherein saidproduct outlet manifold is configured to serve as an internal inletmanifold for also feeding a portion of said product streams to each ofsaid electrode compartments without said portion leaving the stack. 30.An electrodialysis apparatus according to claim 13 wherein, in one ormore compartments, cation exchange material is juxtaposed to cationexchange membrane and anion exchange material is juxtaposed to anionexchange membrane.
 31. An electrodialysis apparatus according to claim13 wherein the cation exchange material is not integral with the cationexchange membrane.
 32. An electrodialysis apparatus according to claim13 wherein the anion exchange material is not integral with the anionexchange membrane.
 33. An electrodialysis apparatus according to claim13 wherein the cation exchange material is not integral with the cationexchange membrane and the anion exchange material is not integral withthe anion exchange membrane.
 34. A process for removing ionized and/orionizable substances from a liquid containing such ionized and/orionizable substances, the process comprising the steps of: (A) providingan electrodialysis membrane stack defining a plurality of dilutingcompartments, alternating with a plurality of concentratingcompartments, whereby each diluting compartment is adjacent to aconcentrating compartment along at least one diluting compartment side,and each concentrating compartment is adjacent to a diluting compartmentalong at least one concentrating compartment side, diluting compartmentand concentrating compartment spacers, at least an electrodecompartment, at least an electrode pair for establishing an electriccurrent across said stack, and inlets and outlets for flowing liquids toor from the stack, said stack further comprising a manifold systemcomprising at least a product outlet manifold which is internal to saidstack and intersects a plurality of the compartments, said productoutlet manifold being configured to receive product streams from atleast two diluting compartments in said stack and also to serve as aninternal inlet manifold for feeding effluent portions of said productstreams to at least two concentrating compartments in said stack wherethe manifold intersects such concentrating compartments without saideffluent portions leaving the stack whereby portions of product streamsfrom at least two diluting compartments in said stack flow into andthrough at least two concentrating compartments in said stack in a flowdirection that is substantially opposite to a direction of fluid flow insaid diluting compartments; and, (B) flowing said liquid into said stackand applying electrical current to said stack.
 35. A process accordingto claim 34 further comprising the steps of providing cation exchangemembranes and anion exchange membranes and winding them together inspiral form, together forming spiral diluting compartments and spiralconcentrating compartments.
 36. A process according to claim 35 furthercomprising the steps of providing liquid entrances and exits arranged toeffect flow of liquid in said spiral diluting compartments inwardly in aspiral or alternatively outwardly in a spiral, and of establishing flowof liquid in said compartments inwardly in said spiral or alternativelyoutwardly in said spiral.
 37. A process according to claim 35 furthercomprising the steps of providing spiral diluting compartments defininga central axis, providing one or more liquid entrances and exitsarranged to effect flow of liquid in said spiral diluting compartmentsin a direction substantially parallel, or alternatively anti-parallel,to said axis, and periodically flowing liquid in said compartmentsparallel, or alternatively anti-parallel, to said axis.
 38. A processaccording to claim 34 further comprising the steps of providing anelectric current switch mechanism for reversing electrical currentdirection across said stack and periodically using said switch mechanismfor reversing electrical current direction.
 39. A process according toclaim 34 further comprising the steps of providing a fluid flow controlmechanism for periodically reversing flow direction in dilutingcompartments and in concentrating compartments, and periodically usingsaid fluid flow control mechanism for reversing flow direction indiluting compartments and in concentrating compartments.
 40. A processaccording to claim 34 further comprising the steps of: (a) providing indiluting compartments of said stack ion exchange material juxtaposed toa cation exchange membrane and to an anion exchange membrane, at leastsaid ion exchange material juxtaposed to said anion exchange membranecomprising anion exchange material, said anion exchange material locatedin regions adjacent to diluting compartment entrances being effective toremove free and combined (available) carbon dioxide from liquid enteringsaid diluting compartments when such combined carbon dioxide issubstantially only bicarbonate; (b) flowing liquid into said dilutingcompartments wherein said liquid includes free and/or combined carbondioxide; and (c) removing free and combined carbon dioxide from saidliquid at the diluting compartment entrances using said anion exchangematerial.
 41. A process according to claim 34 further comprising thesteps of providing at least one segmented electrode controlled by anelectric current control system responsive to the ionic load fed to saiddiluting compartments, and controlling electrical currents to said atleast one segmented electrode using said control system.
 42. A processaccording to claim 34 further comprising the steps of: (a) providing anelectric current regulating system for optimizing electric current; (b)determining dependency of R_(prod) on the quantity I/q(c_(in)−c_(out)),where R_(prod) is a measure of the electrical resistance of product ofdiluting compartments in said stack, I is a measure of electricalcurrent applied to said stack, q is a measure of flow rate in saiddiluting compartments, c_(in) is a measure of ionized and/or ionizablespecies per unit volume in liquid influent to said dilutingcompartments, c_(out) is a measure of ionized and/or ionizable speciesper unit volume in liquid effluent from said diluting compartments; (c)determining any substantial inflection in said dependency; and, (d)operating said stack at one or more values of I/q(c_(in)−c_(out)) whichresult in values of R_(prod) greater than the value of R_(prod) at suchinflection.
 43. A process according to claim 34 further comprising thesteps of establishing a predetermined removal of silica and/or boricacid in diluting compartments of said stack, determining the ionic loadfed to said diluting compartments, and controlling electrical currentbased on said ionic load at levels effective to maintain suchpredetermined removal of silica and/or boric acid.
 44. A processaccording to claim 34 further comprising the steps of establishing indiluting compartments of said stack predetermined regions for removal offree and combined carbon dioxide, determining the ionic load fed to saiddiluting compartments, and controlling electrical current based on saidionic load at levels effective to maintain removal of free and combinedcarbon dioxide within said predetermined regions.
 45. A processaccording to claim 34 wherein said product outlet manifold which isinternal to said stack comprises a set of aligned apertures in themembranes comprising said membrane stack.
 46. A process according toclaim 34 wherein, in one or more compartments, cation exchange materialis juxtaposed to cation exchange membrane and anion exchange material isjuxtaposed to anion exchange membrane.
 47. A process according to claim34 wherein the cation exchange material is not integral with the cationexchange membrane.
 48. A process according to claim 34 wherein the anionexchange material is not integral with the anion exchange membrane. 49.A process according to claim 34 wherein the cation exchange material isnot integral with the cation exchange membrane and the anion exchangematerial is not integral with the anion exchange membrane.